Examine source code of MDN Uptime

{
  "manifest_version": 2,
  "name": "MDN Uptime",
  "description": "This addon shows the time for any MDN website to come up",
  "version": "2.1resigned1",
  "browser_action": {
    "default_popup": "index.html",
    "default_title": "Show MDN speed",
    "default_icon": {
      "96": "logo_96.png"
    }
  },
  "background": {
    "scripts": [
      "app/background.js"
    ]
  },
  "icons": {
    "48": "logo_48.png",
    "96": "logo_96.png"
  },
  "permissions": [
    "webRequest",
    "*://developer.mozilla.org/"
  ],
  "browser_specific_settings": {
    "gecko": {
      "id": "{f7a423e4-3a20-4671-9b84-3f82159eb9d3}"
    }
  }
}